Although they are not yet known, there are strong indications that the causes of autism spectrum disorders are biological. It is likely that autism is not caused by a single factor. The evidence includes the following observations.

Autism is often accompanied by other neurological symptoms and associated with other learning difficulties.

By adulthood, about one in three persons with autism will have had at least two epileptic seizures.

Most autistic children show unusual responses to sensory stimuli of any kind and have what have been called movement differences.

Some 30 to 50 per cent of autistic children have abnormally high levels of serotonin, the chemical that transmits signals in nerve cells.

The higher incidence of autism in families points to a genetic aspect.

There is evidence that at least one type of autism is an immune-system dysfunction, its onset triggered by viral infections, or by antibiotics and vaccines.

Some autistic children are unable properly to digest certain foods, notably casein and gluten, so that toxins called opioids enter the bloodstream and reach the brain to cause havoc in many sensory and cognitive functions.

http://www.uoguelph.ca/oaar/what_causes.shtm

CARS: Childhood Autism Rating Scale

The following items are a sample of the questions found on the Childhood Autism Rating Scale (Eric Schopler, Robert Reichler, MD, and Barbara Rochen Renner, Western Psychological Services, Los Angeles: 1993), also known as the CARS. This instrument is often used to evaluate young children who may have autistic spectrum disorders.